The invention pertains to wireless communications and, more particularly, by way of example, to methods and apparatus providing multiple user detection for use in code division multiple access (CDMA) communications. The invention has application, by way of non-limiting example, in improving the capacity of cellular phone base stations.
Code-division multiple access (CDMA) is used increasingly in wireless communications. It is a form of multiplexing communications, e.g., between cellular phones and base stations, based on distinct digital codes in the communication signals. This can be contrasted with other wireless protocols, such as frequency-division multiple access and time-division multiple access, in which multiplexing is based on the use of orthogonal frequency bands and orthogonal time-slots, respectively.
A limiting factor in CDMA communication and, in particular, in so-called direct sequence CDMA (DS-CDMA) communication, is the interference between multiple cellular phone users in the same geographic area using their phones at the same time, which is referred to as multiple access interference (MAI). Multiple access interference has an effect of limiting the capacity of cellular phone base stations, driving service quality below acceptable levels when there are too many users.
A technique known as multi-user detection (MUD) is intended to reduce multiple access interference and, as a consequence, increases base station capacity. It can reduce interference not only between multiple transmissions of like strength, but also that caused by users so close to the base station as to otherwise overpower signals from other users (the so-called near/far problem). MUD generally functions on the principle that signals from multiple simultaneous users can be jointly used to improve detection of the signal from any single user. Many forms of MUD are discussed in the literature; surveys are provided in Moshavi, “Multi-User Detection for DS-CDMA Systems,” IEEE Communications Magazine (October, 1996) and Duel-Hallen et al, “Multiuser Detection for CDMA Systems,” IEEE Personal Communications (April 1995). Though a promising solution to increasing the capacity of cellular phone base stations, MUD techniques are typically so computationally intensive as to limit practical application.
An object of this invention is to provide improved methods and apparatus for wireless communications. A related object is to provide such methods and apparatus for multi-user detection or interference cancellation in code-division multiple access communications.
A further related object is to provide such methods and apparatus as provide improved short-code and/or long-code CDMA communications.
A further object of the invention is to provide such methods and apparatus as can be cost-effectively implemented and as require minimal changes in existing wireless communications infrastructure.
A still further object of the invention is to provide methods and apparatus for executing multi-user detection and related algorithms in real-time.
A still further object of the invention is to provide such methods and apparatus as manage faults for high-availability.